Performed heating pm on oil boiler Brushed and vacuumed flue pipe and heat exchanger Replaced oil filter, nozzle, and strainer Checked all electrical connections and safeties Performed combustion and smoke tests While performing the maintenance the customer stated they heard a small amount of air in the heating pipes. After getting customer authorization we bled all zones of any air. Started and tested the system. Had hot water coming back on all zones. When leaving the boiler was at 21psi